1. Field of the Invention
This invention relates to film inspection apparatus, and is more particularly concerned with a light modulating device for reading the surface profile of a moving film and stopping the film in response to an unacceptable surface profile.
2. Description of the Prior Art
Various arrangements are known in the art for inspecting films to detect elongated sprocket holes, sprocket run off or punch, edge breaks and poor splices, including Grunwald et al. U.S. Pat. No. 2,699,676; Grunwald et al. U.S. Pat. No. 2,934,949; Grunwald et al. U.S. Pat. No. 3,613,444; Phillimore U.S. Pat. No. 2,469,608; Menary U.S. Pat. No. 3,180,143 and 3,501,760 and Wallace U.S. Pat. No. 3,778,802. These prior art publications disclose various arrangements for detecting unacceptable flaws or defects in film.
As pointed out in the aforementioned Wallace patent, motion picture film usually consist of an elongate strip of material of a thickness of approximately 5 mils which has a picture track consisting of a series of successive pictures or frames occuping a substantial portion of the width of the film, a sound track adjacent the picture track on one edge of the film, and a series of sprocket holes in the opposite edge of the film to receive the sprocket wheels of a projector in order to provide proper indexing of the pictures as the film is shown. Outboard of the sprocket holes, and of course along the opposite edge of the film, the film has a continuous web or track which defines a continuous edge of the film. After a film has been projected, particularly after the film has been projected many times, or after the film has been used on a faulty projector, it tends to acquire defects which will impair future projections. Among these defects are those set forth above which include: elongated sprocket holes, sprocket run off or punch, where the projector sprocket is run out of registry with the sprocket holes and has indented or embossed the film, sprocket holes torn laterally out to the edge of the film and identified as edge breaks, the film splices which are poorly made and occupy an abnormal length along the film, usually in excess of one-eighth inch, and splices made with pins, paper clips or staples, and breaks the film which may have occurred subsequent to the last previous film inspection.
As also pointed out by Wallace, while these prior arrangements have been generally satisfactory in detecting various types of defects in the film, they have been unsuitable for making a suitable inspection when the film is moved past the inspection device at an extremely high rate of speed, in the order of 1,600 feet per minute. Therefore, in the detection of edge breaks the sensitivity of the detector of the prior art arrangements has not been sufficiently sensitive to permit film inspection speeds in the order of 1,600 feet per minute.
Inasmuch as these prior art publications illustrate and teach the driving and braking of the film in an inspection device and as the driving and braking per se do not form a part of the present invention, except in combination with the detection apparatus disclosed herein, these teachings are incorporated herein by reference and detailed description of this apparatus is eliminated herein for purpose of simplicity and clarity.